The only known literature or patent reference that describes a process that could be considered to slightly or remotely resemble that of the present invention, is U.S. Pat. No. 3,796,778 of Gallacher. The disclosure of the Gallacher patent is directed toward the production of a porous mat for use as an artificial leather or fabric, as a porous substrate in batteries or fuel cells, as an industrial membrane, as a bandage, etc.
Gallacher describes a slow process of mixing microparticles of a low temperature melting point thermoplastic with a second powder composed of high temperature melting point plastic. This mixture is then heated and extensively processed by shear for many minutes, usually using a rolling mill. Gallacher fails to teach any adjustment of operating conditions affecting the applied heat, shear, and compression. He discloses a slow, intensive shearing process at low temperatures and very high concentrations of binder resin.
Following the production of fibers from the binder particles, Gallacher dissolves the non-fiberized plastic primary particle component to leave a network of binder fibers with high porosity and tensile strength. This procedure is specifically designed for the production of high-porosity structures composed of binder-resin fibers. In addition, Gallacher utilizes a high binder resin content because he is seeking dense fibrous mats.
Gallacher, in his specific examples, applies compression only after the formation of the fibers to produce a thinner sheet of material. He does not apply intense pressure in conjunction with shear only after heating. Moreover, it takes many minutes for the Gallacher process to produce a fibrous structure which he then compacts to make thinner.
Gallacher focuses on the production of fibrous mats of binder resin and neglects the nonfiberized particles. His interest is directed to primary particles that can be dissolved from the structure to leave the fibrous mat of binder resin. The use of solvents to remove the primary particles is an environmentally unacceptable practice. The disclosure is confined to high-shear rolling, milling, or extrusion processes and to thin structures because his preferred methods and equipment cannot produce bulk shapes or bulk extrusions, a major limitation of his process.
Gallacher does not suggest the retention of the primary particles to form unique materials. Accordingly, he could not produce any of the products or end items, such as ion-exchange cartridges, stainless steel filter media, membrane supports, molded filters, etc. that are potential products of the subject invention, which are produced utilizing temperatures substantially higher than the melting point of the binder resin. Gallacher recommends temperatures well below these temperatures and employs, instead, temperatures approximately equal to the melting point of the binder resin.
In addition to Gallacher, another U.S. patent discloses a "point bonding" technique. This is U.S. Pat. No. 4,664,683 of Degen and Gsell. However, the process disclosed in that patent does not produce the unique structures characteristic of the present invention and the applied temperatures, pressures, and shear during the process are entirely different from those required to produce the continuous polymeric phase and forced point-bonding of the present invention. The Degen et al. technology is basically a low temperature diffusion bonding process.
U.S. Pat. No. 4,664,683 is specific to the point bonding of Whetlerite, or ASC, activated carbon, which is primarily used for defense against chemical warfare agents. Similar impregnated carbons are sometimes used for protection of industrial workers against low molecular weight toxic gases. The process disclosed by such patent appears to be essentially identical to that used by Norit Carbon of the Netherlands for the production of Norithene (a registered trademark of Norit Carbon), which is composed of point-bonded activated carbon that is formed into sheets.
The levels of compression disclosed by Degen et al. are exceedingly low, 0.3-10 psi (0.21-0.703 kg/cm.sup.2) most preferred maximum 40 psi (2.91 kg/cm.sup.2). Accordingly, it describes process conditions well outside the range of compression utilized in the present invention, which would be 400-1000 psi (28.1-70.31 kg/cm.sup.2) for granular materials (i.e. 10-50 mesh) and approximately 8,000 psi (562.48 kg/cm.sup.2) or more for powders (typically, 100-600 mesh). Without such higher pressures, the binder resins are not activated and the novel structures produced by the current invention are not obtained.
Degen et al. U.S. Pat. No. 4,664,683 also describes a process using a temperature of approximately 275.degree. F. (135.degree. C.), which is generally below the temperature required in the subject invention to achieve the desired novel structures. Formation of a novel continuous polymer phase or forced point-bonding, according to the present invention, with the lowest melting point resin available, ethylene-vinyl acetate copolymer (EVA), usually occurs at 145.degree. C. for even small bulk shapes and is optimal in the range of 165-210.degree. C. The temperatures required by the process of the subject invention are therefore substantially higher than required for diffusion bonding processes such as that described by Degen et al., even for the binder resin having the lowest melting point. Degen et al. teach the use of temperatures only sufficient to produce a softening of the binder because they are seeking a point bond and are not seeking a more dramatic conversion of the thermoplastic binder into a different physical form.
According to the Degen et al. disclosure, a low level of compression is applied to the activated carbon and it is then heated. In their process, the mass of carbon is slightly compressed and consolidated prior to the application of heat and the formation of point bonds. Therefore, there is no potential application of shear during heating, which has been found to be a critical condition for the process of the subject invention. The process of the present invention requires the simultaneous application of shear and compression following the heating of the mixture. The point bonding process of Degen et al. reverses this sequence by compressing the mixture while cold and then applying heat to raise the temperature to a level insufficient to produce the formation of a continuous polymeric phase or a forced point-bonded structure.
The point-bonding process described by Degen et al. cannot be applied to fine powders because of the rapid escalation in the quantity of binder resin powder required to achieve point bonding in powders. This is due to the enormous amount of powder surface to be bonded. The Degen et al. process is therefore limited to coarse granular carbons, while the current invention can be efficiently applied to powders composed of particles as small as one micron in diameter.
Another prior art patent, assigned to the same assignee as the Degen et al. patent, is U.S. Pat. No. 4,687,573 of J.D. Miller and M.G. Verrando. This patent describes the immobilization of sorbents to prevent the fluidization of particles within adsorbent systems. One of the primary limitations of all sorbent systems is that they must be operated below the velocity that would result in the fluidization of the sorbent and a loss of staging and possibly a substantial increase in attrition. Miller et al. disclose a system wherein a sorbent is immobilized to substantially eliminate this limitation. The method of immobilization bears no resemblance to the process of the present invention.
U.S. Pat. No. 3,864,124 of E.J. Breton, J.D. Wolf, and D. Worden is typical of a number of similar patents disclosing the use of polytetrafluoroethylene (PTFE) to immobilize a non-fiberizing material. However, none of these are related to the process of the present invention. Many different products are produced using one of several variations on the method described in this patent. All are based upon the immobilization of powders within a matrix of PTFE fibers that are produced by in-situ fibrillation. PTFE is so expensive as to preclude this technique from all but the highest value-added products. Also, PTFE is unique in that it fibrillates without heating or applying substantial compression, but by shear and pulling alone. Such a process is used to produce membranes, porous carbon battery electrodes, and other materials.
The foregoing process using PTFE is complex and time consuming and involves the evolution of fine fibers by mechanically working and shearing a mixture of PTFE and particles. PTFE produces highly toxic fumes when brought to metal sintering temperatures and therefore represents a significant environmental and health problem if used to immobilize metal particles. The PTFE binder particles are 50-560 micrometers, which are generally much larger than the binder particles used in the process of the present invention. The time required to convert these particles into fibers is substantial in comparison to the process of this invention, which is often complete in less than one second. The PTFE method can only produce a thin sheet product and cannot generally be used to form thick structures. It cannot be used to produce extruded or molded products.
Similar methods for the production of immobilized materials using cold-worked PTFE are disclosed in U.S. Pat. No. 4,379,772 of F. Solomon and C. Grun for the production of hydrophobic battery electrodes, and in European Patent Application 0056724 (P. Bernstein et al., MPD Technology Corporation, 28 July 1982) for the immobilization of hydride-forming particles suitable for the storage of hydrogen.
One of the methods reported in the literature for the production of inorganic hollow fibers is the procedure described in U.S. Pat. Nos. 4,222,977 of Dobo and 4,329,157 of Dobo and Graham. Dobo describes a method involving the dispersion of a fine powder such as a metal oxide or metallic powder into a fiber-forming polymer and the extrusion of this polymer-based suspension through a conventional hollow fiber extruder. Sintering the resulting structure results in the production of a fine porous or nonporous metallic hollow fiber. Although the method appears to have been demonstrated in the laboratory, it appears not to have been put into production. The method described by Dobo is not related to the process of this invention.
In summary, there is no known information indicating that the process of this invention has been previously described in the prior art. The process of this invention, and the products produced by the process, are therefore believed to be entirely new and original.